Electrooptic devices, such as photodetectors, that respond to modulation at mm-wave frequencies are becoming increasingly important for very high speed digital fiber-optic communications links, and for optical transmission and signal processing of microwave and radar signals. Unfortunately, it has become increasingly difficult to measure the frequency response of these devices once their bandwidth exceeds 20 GHz primarily because it is difficult to separate the response of the detector from the response of the measurement system used to analyze the detector. Although a number of methods for measuring frequency response have been successfully implemented, none has been completely satisfactory as of yet. Conventional measurement systems have used directly modulated high frequency sources [Blauvelt et al., "Fabrication and Characterization of GaAs Schottky Barrier Photodetectors for Microwave Fiber Optic Links", Appl. Phys. Lett. 45(3), pp. 195-196, 1984], external modulators (Blauvelt et al., supra), picosecond optical pulses [C. A. Burrus et al., "Improved Very-High-Speed Packaged InGaAs PIN Punch-Through Photodiode", Elect. Lett. 21(7), pp. 262-263 (1985)], beating two semiconductor lasers together [L. Piccari and P. Spano, "New Method for Measuring Ultrawide Frequency Response of Optical Detectors", Elect. Lett. 18(3), pp. 116-118 (1982)], and interferometrically demodulating the FM sidebands of a modulated semiconductor laser [E. Eichen and A. Silletti, "Bandwidth Measurements of Ultra-High Frequency Optical Detectors Using the Interferometric FM Sideband Technique", J. of Lightwave Tech. LT-5(10), pp. 1377-1381 (1987)]. The above-mentioned systems, however, are not suitable for measuring photodetector frequency response for the following reasons: swept frequency measurements by Blauvelt et al. require either an intensity modulated laser or an external modulator, both with a known response greater than that of the detector; time domain measurements by Burrus et al. require picosecond optical pulses which are easily affected by sampling and computational errors; and the beat frequency method as taught by Piccari and Spano requires two narrow linewidth semiconductor lasers with excellent temperature control and no optical feedback. Another method, disclosed by T. Andersson et al. in "Temporal and Frequency Response of Avalanche Photodiodes From Noise Measurements", Appl. Optics 19(20), pp. 3496-3499 (1980), uses the shot-noise spectrum from which to derive the frequency response of the photodetector. Disadvantageously, this method requires an amplification stage due to the extremely low signal levels obtained during the measurement of the shot noise frequency spectrum.